The term user equipment (UE for short) in this application means the (potentially moving) piece of equipment that communicates with the fixed network of radio transceivers called base stations in a mobile communications network. In different standards or circumstances it may be named differently, such as a terminal. The term user equipment used herein is, however, meant to encompass any such differently named equipment that serves the purpose mentioned above.
Likewise, nodeB or eNodeB (eNB for short) is a term often used within the Long Term Evolution (LTE) mobile communications network framework. However, the use of these terms in this application should not be interpreted to restrict the invention to the LTE framework. Rather, they should be interpreted in the broader sense as a radio basestation in any suitable mobile communications system.
In a wireless communication system it is well known that utilizing linear precoding at the transmitter side with multiple antennas can improve the performance substantially. FIG. 1 shows the block diagram related to downlink (DL) transmission with linear precoding in 3GPP LTE (Long Term Evolution) Rel-8/9/10 system.
Antenna ports shown in FIG. 1 convey the multiple transmitted signals that are spatially separable at the receiver. An antenna port is defined by its associated reference signal (RS). In practice, an antenna port may be implemented either as a single physical transmit antenna, or a combination of multiple physical antenna elements. Some proprietary operation might be implemented prior to mapping antenna port onto physical antenna elements. In either case, the signal transmitted from each antenna port is not designed to be further deconstructed by the receiver.
Layers are defined as strings or vectors of the modulated symbols to be transmitted on different antenna ports.
Channel State Information
In the connection of this application, knowledge of the term CSI may be useful. CSI is short for Channel State Information. There are basically two levels of CSI, namely instantaneous CSI and statistical CSI.
Instantaneous CSI (or short-term CSI) means that the current channel conditions between a transmitter and a receiver are known, which can be viewed as knowing the impulse response of a digital filter. This gives an opportunity to adapt the transmitted signal to the impulse response and thereby optimize the received signal for spatial multiplexing or to achieve low bit error rates.
Statistical CSI (or long-term CSI) means that a statistical characterization of the channel is known. This description can include, for example, the type of fading distribution, the average channel gain, the line-of-sight component, and the spatial correlation. As with instantaneous CSI, this information can be used for transmission optimization.
The CSI acquisition is practically limited by how fast the channel conditions are changing. In fast fading systems where channel conditions vary rapidly under the transmission of a single information symbol, only statistical CSI is reasonable. On the other hand, in slow fading systems instantaneous CSI can be estimated with reasonable accuracy and used for transmission adaptation for some time before being outdated.
In practical systems, the available CSI often lies in between these two levels; instantaneous CSI with some estimation/quantization error is combined with statistical information.
Linear Precoding
The purpose of linear precoding is to match the instantaneous channel properties and allows coherent combining of the transmitted signals at the receiver side and reducing the inter-layer interference thereby improving received SINR (Signal to Interference and Noise Ratio). Different linear precoding schemes have been implemented in IEEE 802.16-2005 and in LTE Release-8/9/10 standards.
One of these well-known linear precoding schemes is known as closed loop precoding i.e. channel-dependent precoding. In order to perform closed loop precoding in the downlink, the receiver, also known as the user equipment (UE), has to estimate the corresponding channel and report the estimated channel information to eNB for precoding. In order to reduce the overhead of directly reporting quantized estimated channel, a codebook consisting of a number of precoding matrices roughly representing the true channel may be defined. The UE uses the knowledge of estimated channel for selecting an appropriate precoder (PMI) from the defined codebook, transmission rank and estimated CQI (Channel Quality Information). These fundamental values are the CSI (Channel State Information) mentioned above, and are fed back to the transmitter i.e. NodeB. Once receiving this information NodeB performs scheduling for downlink transmission for the UE.
For example, LTE Rel.8 standard uses a precoding matrix codebook consisting of 64 matrices for up to 4 transmit antennas and different ranks. Therefore, UE can feed back the appropriate precoder to represent the estimated DL channel using only 6 information bits instead of a large number of channel quantization bits.
It is important to notice that for closed loop precoding, the selected precoding matrix based on instantaneous channel knowledge at time t will be used for precoding at time T (T>t) due to the processing time needed for channel measurement, precoding matrix selection and the propagation time for reporting precoding matrix or signaling. As the channel is time varying, the appropriate selected precoding matrix at time t may be obsolete and not match the channel at time T when the channel varies fast in time such as high mobility scenarios. The result is that the modulation and coding scheme (MCS) determined by the CQI (Channel Quality Information) corresponding to the reported precoding matrix at time t will not be suitable for transmission at time T, which may degrades the performance. Hence, in order to achieve the closed loop precoding gain, the channel over time should vary slowly, i.e. low mobility.
Closed loop precoding schemes are used extensively in LTE Rel-8/9/10. For example PDSCH (Physical Downlink Shared Data Channel) transmission modes 4, 8 and 9 known as spatial multiplexing use closed loop techniques.
For above mentioned reasons and for specific scenarios such as high mobility, open loop precoding known as channel-independent precoding scheme is also used in LTE Rel-8 which is known as transmission mode 3.
A major difference between the closed loop and open loop scheme is that in the open loop scheme the used precoding matrices are predefined and there is no precoding matrix selection. As both eNB (transmitter) and UE (receiver) knows the predefined precoding matrices, the UE only feeds back CQI and selected rank. In the case of open loop precoding this CQI corresponds to an average CQI over the channel. Average CQI over the channel is computed by averaging CQI over a set of the predefined precoders known both at the UE and eNodeB side.
As described above, the antenna port conveying the transmitted signal is defined by its associated Reference Signal (RS). The RS is used to estimate the channel for measurement and demodulation.
In LTE Rel-8 DL transmission, Cell-specific Reference Signals (CRS) are defined for both measurement and demodulation. The CRSs are broadcasted and common to all the UEs in the cell, and each CRS defines one antenna port. The number of antenna ports for eNB can be configured as 1, 2 and 4, and the corresponding antenna port numbering is {0}, {0, 1} and {0, 1, 2, 3}.
In LTE Rel-8, for PDSCH (Physical Downlink Shared Channel) transmission using linear precoding, the output of precoding is mapped into a number of antenna ports defined by CRS as illustrated in FIG. 2 (assuming four antenna ports).
To perform demodulation in this case, the UE needs to know the transmission channel between the antenna ports and the receiver, and also the used precoding matrix. The transmission channel can be obtained by performing channel estimation based on CRS. The used precoding matrix is explicitly signalled in the physical downlink control channel (PDCCH) in the form of signalling PMI (Precoder Matrix Indication).
In LTE Rel-10 DL transmission, the Reference Signals (RSs) for measurement and demodulation are decoupled to CSI reference signals (CSI-RS) and User Equipment (UE)-specific reference signals. The CSI-RS is used for measurement, which is cell-specific and is common to all the Rel-10 UEs in the cell. Compared to the CRSs in LTE Rel-8, the CSI-RS has lower density in the frequency and time domain. The UE-specific reference signal is used for PDSCH (Physical Downlink Shared Channel) demodulation, which is only transmitted in the scheduled resource block (RB) for a certain UE. UE-specific reference signal is denoted as DM-RS (Demodulation Reference Signal).
For PDSCH demodulation using DM-RS, the same precoding scheme is applied to both DM-RS and PDSCH. The DM-RS is used to estimate the precoded channel, and therefore the number of DM-RSs would be equal to the number of layers of PDSCH or rank. As each DM-RS defines one antenna port, the number of DM-RS antenna ports used for PDSCH transmission would depend on transmission rank. Up to eight DM-RS antenna ports {7, 8, 9, 10, 11, 12, 13, 14} are defined to support up to eight layers transmission in LTE Rel-10.
As the DM-RS is precoded, UE can obtain the transmission channel and the used precoding matrix together by performing channel estimation based on DM-RS. Hence the PMI information does not need to be included in the PDCCH. In this case, PDSCH is directly mapped onto the antenna ports defined by DM-RS as illustrated in FIG. 3, which is the case of rank 4 transmission corresponding to transmission via antenna ports {7, 8, 9, 10}. It is important to notice that the precoder in this case can be a proprietary precoder and is not defined in the specification.
RS pattern defined for four antenna ports (7˜10) and related DM-RS in normal sub-frame are shown in FIG. 4.
In Rel-8 an open loop precoding scheme which is known as transmission mode 3 is designed to be used in high mobility scenarios, see TS36.213, “Evolved Universal Terrestrial Access (E-UTRA); Physical Layer Procedures”, Release 10, referenced as [2] below. This transmission mode uses open loop transmit diversity scheme (TxD) when the transmission rank is 1, otherwise, it uses large delay CDD (Cyclic Delay Diversity) precoding.
For large delay CDD, precoding is defined by:
      [                                                      z                              (                0                )                                      ⁡                          (              i              )                                                                                      z                              (                1                )                                      ⁡                          (              i              )                                                            ⋮                                                                z                              (                                  P                  -                  1                                )                                      ⁡                          (              i              )                                            ]    =            W      ⁡              (        i        )              ⁢          D      ⁡              (        i        )              ⁢          U      ⁡              [                                                                              x                                      (                    0                    )                                                  ⁡                                  (                  i                  )                                                                                                                          x                                      (                    1                    )                                                  ⁡                                  (                  i                  )                                                                                        ⋮                                                                                            x                                      (                                          v                      -                      1                                        )                                                  ⁡                                  (                  i                  )                                                                    ]            
Where P denotes the number of antenna ports, v denotes transmission rank. [x(0)(i)x(1)(i) . . . x(v-1)(i)]T represents the block of data vectors from the layer mapping according to FIG. 1 and [z(0)(i)z(1)(i) . . . z(P-1)(i)]T is the block of vectors to be mapped on resources on each of the antenna ports. Precoding matrix W is of size P×v and i=0, 1, . . . , M with M being the number of modulation symbols per layer. The matrix D(i) is supporting cyclic delay diversity and matrix U is of size v×v. These matrices are specified and are given in reference TS36.211, “Evolved Universal Terrestrial Access (E-UTRA); Physical Channels and Modulation”, Release 10 (referenced as [1] below) for different number of layers v.
In the case of 4 antenna ports, the receiver or UE assumes that the transmitter or eNodeB cyclically assigns different precoders to different vectors [x(0)(i)x(1)(i) . . . x(v-1)(i)]T on PDSCH. A different precoder is used every v vectors. Different precoders are selected from Table 6.3.4.2.3-2 of [1]. Due to the precoder cycling, several precoding matrices are used in one RB.
The precoder cycling achieves an average Channel Quality Information (CQI) over a number of different channels by using different precoders. Therefore, this method is more robust to channel variation than that of the closed loop precoding which represents only the best instantaneous channel and the best related precoder.
On the top of precoder cycling done by the use of several W(i), layer permutation is also performed for each precoder by using matrices D(i) and U. The layer permutation for each precoder enables the two codewords (when there is more than one layer transmission) to have the same CQI, and therefore reporting only one CQI is sufficient which reduces CQI feedback overhead.
In this open loop transmission mode, Cell-specific Reference Signals (CRSs) are used to obtain channels corresponding to each antenna port. Moreover, the precoders used for cycling are known according to a predefined rule by both the transmitter (NodeB) and the receiver (UE). The UE will compute the average CQI from the knowledge of channel and set of the predefined precoders.
This prior art is not applicable for the case of using UE-specific reference signals for demodulation, where there is no signalling to determine precoding matrices. Moreover, in this described prior art, cell-specific reference signals (CRS) are used to estimate the transmission channel.
Another prior art can be found in R1-110338, “Remaining details of feedback for TM9”, 3GPP RAN1 #63b, Qualcomm (referenced as [4] below). This proposal is an extension of open loop precoding defined in Rel-8 which is described above. This contribution is a first attempt to use UE-specific reference signals to achieve precoder cycling suitable for open loop MIMO.
More precisely, in [4], a precoder W(iRB) (which is assumed constant per RB) is applied on UE-specific reference signals specified for Rel-10 (DM-RS). This can be written as the following:
      [                                                      z                              (                0                )                                      ⁡                          (              i              )                                                                                      z                              (                1                )                                      ⁡                          (              i              )                                                            ⋮                                                                z                              (                                  P                  -                  1                                )                                      ⁡                          (              i              )                                            ]    =            W      ⁡              (                  i          RB                )              ⁡          [                                                                  r                                  (                  0                  )                                            ⁡                              (                i                )                                                                                                        r                                  (                  1                  )                                            ⁡                              (                i                )                                                                          ⋮                                                                              r                                  (                                      v                    -                    1                                    )                                            ⁡                              (                i                )                                                        ]      
In the above equation W(iRB) denotes the RB-specific precoder and v denotes transmission rank. [r(0)(i)r(1)(i) . . . r(v-1)(i)]T denotes the data for different layers. As per previous contribution, average CQI is assumed being evaluated over a set of predefined precoders which are known to both receiver (UE) and transmitter (NodeB) and the channel is estimated from cell-specific CSI-RS. It is also assumed that the set of predefined precoders are cyclically used in different RBs.
It is known from large delay Cyclic Delay Diversity (CDD) precoding of Rel-8 that the matrix D(i) and U supports layer permutation which provides the same CQI for both two codewords for larger than rank one transmission and consequently reducing feedback overhead. Therefore, in another prior art R1-110976, “Standardized UE-RS based open loop SU-MIMO”, 3GPP RAN1 #64, Intel (referenced as [3] below), which is an extension of [4], it is proposed to use not only precoding W(iRB) on UE-specific reference signals but also per subcarrier large delay CDD (D(i)U). Corresponding downlink (PDSCH) transmission can be written as the following:
      [                                                      z                              (                0                )                                      ⁡                          (              i              )                                                                                      z                              (                1                )                                      ⁡                          (              i              )                                                            ⋮                                                                z                              (                                  P                  -                  1                                )                                      ⁡                          (              i              )                                            ]    =            W      ⁡              (                  i          RB                )              ⁢          D      ⁡              (        i        )              ⁢          U      ⁡              [                                                                              x                                      (                    0                    )                                                  ⁡                                  (                  i                  )                                                                                                                          x                                      (                    1                    )                                                  ⁡                                  (                  i                  )                                                                                        ⋮                                                                                            x                                      (                                          v                      -                      1                                        )                                                  ⁡                                  (                  i                  )                                                                    ]            
As mentioned before, precoding operation is predefined for both UE and eNodeB. However, UE does not know how D(i) is used for different resource elements.
As introduced in the background, Downlink (DL) precoding using antenna ports with UE-specific reference signals is only optimized for closed loop i.e. low mobility. Further evolution of Rel.10 for high mobility scenarios will reuse UE-specific reference signals to guarantee the backward compatibility, simplifying the design and keeping the signalling overhead lower. Hence the problem to resolve is how to improve the precoding performance at high mobility using antenna ports defined by UE-specific reference signals without signalling the used precoders while achieving the best performance.
Thus, there is always a need for improvements and thus the present application sets out to provide a new scheme with improved characteristics.